1. Field of the Invention
The present invention relates to a cam mechanism which can be used in a lens barrel, e.g., a photographing lens of a camera. The cam mechanism includes a cam ring on which at least one cam rib is formed.
2. Description of the Related Art
A cam mechanism including a cam ring and a follower ring which are concentrically arranged, wherein at least one cam rib extending non-linearly is formed on an inner or outer peripheral surface of the cam ring while a corresponding at least one pair of cam followers are formed on an outer or inner peripheral surface of the follower ring, is known in the art. The present invention relates to this type of cam mechanism. This type of cam mechanism will be hereinafter referred to as a “cam-rib-incorporated cam mechanism”. In the cam-rib-incorporated cam mechanism, if the primary cam diagram of the cam rib has only a circumferential component (i.e., if the primary cam diagram extends only in a circumferential direction of the cam ring), the pair of cam followers must be positioned apart from each other in the axial direction of the follower ring to hold the associated cam rib between the pair of cam followers. In other words, a pair of cam followers cannot hold the associated cam rib therebetween if positioned apart from each other only in a circumferential direction of the follower ring. The follower ring is usually guided linearly along the optical axis without rotating. Accordingly, rotating the cam ring forward and reverse causes the follower ring to move forward and rearward in the axial direction thereof.
In the cam-rib-incorporated cam mechanism, it is conventionally the case that the shapes of the opposite side surfaces of the cam rib are determined by a cam track delineated by paths of the associated pair of cam followers. Namely, the respective positions of the pair of cam followers are determined first, and thereafter the area between the paths of the associated pair of cam followers is determined as a representation of the shape of the cam rib.
In the cam-rib-incorporated cam mechanism, in the case where the ratio of the circumferential component of the cam rib to the axial component of the cam rib varies greatly (e.g., the case where the cam rib includes both an inclined rib section which is inclined to a circumferential direction of the cam ring at an angle of 45 degrees and a circumferential rib section which is not inclined to the same circumferential direction), the width of the cam rib varies greatly, which inevitably produces a narrow rib portion and a very wide rib portion in the cam rib. When the cam rib is molded out of synthetic resin to be integral with the cam ring, a portion of the cam rib can cause a shrinkage cavity to occur, which is produced if a liquefied synthetic resin cannot be injected sufficiently into a molding cavity for molding the cam rib, which has a greater width as viewed from outside (inside) of the cam ring. This deteriorates accuracy of the shape of the cam ring. In addition, in the case where the cam-rib-incorporated cam mechanism is incorporated in a lens barrel such as a photographing lens, an increase in width of the cam rib increases the axial length of the cam ring. This is disadvantageous in regard to reducing the length of the lens barrel.